Flexible expanded graphite is a well-known material used in a variety of industrial, commercial and domestic applications because of its chemical inertness and unique anisotropic electrical and thermal conduction properties. Applications for flexible expanded graphite include chemical and automotive gasketing, valve stem and pump packings, seals, electromagnetic and thermal radiation shielding, furnace linings, heat sinks, and the like.
Flexible expanded graphite is produced from naturally occurring graphite flake that is chemically treated to form a compound intercalated with and between layers of the graphite structure. The “intercalated” graphite particles are then exposed to high temperature for a short period of time. The result is an over 80-fold expansion in the volume between the graphite layers. This expansion (“exfoliation”) produces worm-like or vermiform structures with dendritic rough surfaces that can then be compressed into sheet material. The density and thickness of the sheet material can be varied by controlling the degree of compression. For example, the density of the compressed sheet material can range from about 5 to about 137 lbs./ft3, which is near the theoretical density of graphite. For practical applications, such as those described above, flexible expanded graphite foil sheet is commercially available in densities ranging from 50 to 90 lbs./ft3 and thicknesses of 3 to about 60 mils, with a thickness of 15 mils the most common.
Flexible expanded graphite sheet has a relatively high resistivity along its length and width, and excellent heat conducting and electrical conducting properties that are well suited for use in low voltage heater applications. However, the usefulness of this material for high voltage heater applications (e.g., 110, 220 or 440 volts alternating current, VAC) has been limited because of the unavailability of flexible expanded graphite heating elements having a sufficiently high electrical resistance.
A variation in the length, width and/or thickness of flexible expanded graphite sheet can change, by a large magnitude, the electrical resistance and, consequently, the amount of electric current that will flow through the material. For a given length and width, an increase in the thickness of a flexible expanded graphite sheet results in a decrease in the electrical resistance and a higher current flow. For high voltage applications that require very high resistance, it is therefore desirable to use a flexible expanded graphite heating element that is as thin as possible. However, commercially produced flexible expanded graphite sheet that has a minimum thickness of 3 mils does not provide a sufficiently high electrical resistance for most high voltage heater applications.